A gravity compensation mechanism which has been developed so far largely employs a method of using of a weight and a method of using a spring and a wire.
In particular, in the industrial equipment, an appropriate weight is installed at a side opposite to a point of application of a force in order to maintain the center of gravity and compensate for the equipment's own weight.
However, such a method increases the entire mass and volume of the mechanical parts increases, thus leading to a difficulty in using it in robot arms in which the design for lightweightness and compactness and the collision safety are important.
In an attempt to solve this problem, a method of using a compression force and a tensile force of the spring has been proposed.
This method employs a spring repulsive force generated when the wire connected to the link stretch or compress the spring at the time of the rotation of the link in order to compensate for the weight of the link.
Such a method employing the spring enables the gravity compensation mechanism to be manufactured in a relatively small volume and weight compared to a conventional method.
However, the above method is highly likely to cause a problem in that when the gravity compensation mechanism is used for a long period of time, the wire may be damaged, such as being stretched or cut out, thus leading to a reduction in the performance of the gravity compensation mechanism, and resulting in a threat to safety.
Therefore, there is a need for the development of a novel multi-degree of freedom (MDOF) gravity compensation device having high reliability and durability, which can substitute for the conventional wire-based gravity compensation device in order to realize the practical use and commercialization of the gravity compensation device which has been developed.
The basic operation concept of the counterbalancer provided in the present invention is as shown in FIG. 1.
As shown in FIG. 1(a), when a general link having one degree of freedom is rotate by θ, the following gravitational torque Tg is applied to a joint:Tg(θ)=mg/sin θwhere m is a mass of the link and l is a distance to the center of gravity.
Thus, in general, a motor and a speed reducer which can sufficiently support the gravitational torque applied to the joint is required to be used in order to rotate the link or maintain the posture of the link. In case of this link, as shown in FIG. 1(a), a spring can be interposed between a reference plane and the link so as to compensate for the gravitational torque applied to the joint at a certain position by the link's own weight.
However, the gravitational torque varies depending on the rotational displacement of the link as show in FIG. 1(b), and thus a gravity compensation device is needed in which a required torque can be zero in all the displacements by properly compensating for the torque varying depending on the angle in order to perform complete gravity compensation.
In the case where such a gravity compensation device is mounted at the robot arm, the gravitational torque applied from the weight of a robot is offset to remarkably reduce a torque required for the movement of the robot so that the robot arm can be configured only with a low-capacity motor and a speed reducer.
Various kinds of gravity compensation devices employing the spring were developed to obtain such gravity compensation, but a problem involved in weight and reliability has been posed continuously.
In order to solve this problem, the present inventors have proposed a configuration of a gravity compensation mechanism using a wire and a gravity compensation mechanism that matches a reference point relative to the ground surface during the rotation of the joint so as to minimize the required torque needed to perform the gravity compensation, which are disclosed in Korean Patent Application No. 10-2011-0092171.
In FIGS. 2 to 4, there is disclosed a multi-link structure according to the prior applications by the present inventor, in which a stable gravity compensation structure is formed with respect to pitch joints (i.e., pitches 1, 2 and 3) parallel with the ground surface through a wire, a spring, a spring block, and a double parallelogram mechanism (see FIGS. 2 and 3).
However, in the case where a link of a serial connection structure is pivotally rotated with respect to a yaw joint perpendicular to the pitch joint of the pitch so as to be orientated at 90 degrees to the ground surface, i.e., a rotary shaft is arranged vertical to the ground surface, a rotational movement of the link around the pitch joint occurs so that even in the case where the pitch joint does not require a counterbalancing torque for gravity compensation, a compressive restoring force by the spring is formed due to the structure of a gravity compensation mechanism and a curved parallelogram unit which are mounted at the link, thus leading to occurrence of a phenomenon in which a required torque increases unnecessarily on a plane horizontal to the ground surface (see FIG. 4).